1. Field of the Invention
The present invention generally relates to laser systems and, more particularly, to a technique for seeding a laser for single longitudinal mode oscillation.
2. Discussion of the Related Art
Laser systems include a cavity wherein axial (or longitudinal) modes oscillate to stimulate output. Depending on the particular requirements of an application, the laser can be controlled to operate in single or multiple modes. In single mode operation, only one axial mode within the cavity oscillates. In multiple mode operation, more than one axial mode oscillates.
To initiate such axial mode oscillation, radiation from an outside source is injected into the laser cavity. Such injected radiation is known in the art as seed radiation. The seed radiation excites one or more of the axial modes within the laser cavity thereby limiting output to only those modes.
Certain processes involving laser systems require the laser to generate a certain narrow frequency bandwidth to operate properly. One such process is non-linear optical phase conjugation. Within the acceptable bandwidth, the conjugation process provides the desired results. Outside the bandwidth, the conjugation process may cause deleterious effects on the laser system and may even lead to destruction of some optical components.
To ensure proper bandwidth generation, the frequency of the seed radiation must match the frequency of one of the axial modes within the laser cavity. This ensures that the laser cavity only emits one frequency or axial mode. The end result is that the laser cavity emits a controlled frequency rather than the normal multitudes of frequencies.
Conventional techniques for matching the seed radiation to a single axial mode rely on single frequency seed radiation sources and some type of resonant feedback control system. The feedback allows the laser cavity or the seed radiation to be adjusted in real time to ensure nearly continuous matching. In one mechanism, feedback is employed to dither the laser cavity length so that the axial modes within the cavity scan for the best coincidence with the external seed radiation frequency. After the best match is found, the laser cavity is locked in place. In another mechanism, feedback is employed to dither the seed frequency to match a fixed laser cavity. In this case, the seed frequency is varied as it is injected until a matched condition is achieved. Thereafter, the seed frequency is fixed.
While the performance of such feedback control based systems is generally satisfactory, it is not always reliable. This is particularly true under demanding environmental conditions. For example, mechanical vibration in aircraft and other mobile platforms can impact the dithering of the cavity or the seed frequency. Attempts to dampen the system from external vibrational influences with very stiff platforms have been only marginally successful at isolating the system. Eventually, the external vibrations negate the attempt to match the frequencies with feedback control.
Therefore, it is an object of the present invention to provide a more robust seeding technique for stimulating single mode oscillation in a laser system so that the system will be impervious to external vibrational influences.